NMR properties of the polar phase of superfluid He-3 in anisotropic aerogel under rotation

TL;DR

This paper investigates the NMR properties of the polar phase of superfluid helium-3 in anisotropic aerogel under rotation, revealing how vortex configurations affect NMR signals and phase transitions.

Contribution

It demonstrates how half-quantum vortices produce a distinct NMR satellite signal, enabling differentiation from single quantum vortices, and explores phase transitions in anisotropic aerogel.

Findings

Half-quantum vortices cause a shifted NMR satellite in strong magnetic fields.

The satellite signal is absent for single quantum vortices.

The polar phase transitions to the axipolar state at lower temperatures.

Abstract

The polar phase of superfluid $^3$He is stable in "nematically ordered" densed aerogel. A rotating vessel with the polar superfluid can be filled either by an array of the single quantum vortices or by an array of the half-quantum vortices. It is shown that the inhomogeneous distribution of the spin part of the order parameter arising in an array of half-quantum vortices in strong enough magnetic field tilted to the average direction of aerogel strands leads to the appearance of a satellite in the NMR signal shifted in the negative direction with respect to the Larmor frequency. The satellite is absent in the case of an array of single quantum vortices what allows to distinguish these two configurations. The polar state in the anisotropic aerogel with lower density transforms at lower temperatures to the axipolar state. The array of half-quantum vortices created in the polar phase keeps its structure under transition to the axipolar state. The temperature dependence of the vortex-satellite NMR frequency is found to be slower below the transition temperature to the axipolar state.